This invention relates to the detection of faults on low voltage distribution cables carrying low frequency alternating electric current. It is particularly applicable to low voltage distribution cables in a power supply system.
Low voltage cable faults can be divided into three groups, -- transitory, non-persistent and permanent. It is believed that many faults appear initially as transitory faults, develop into non-persistent faults and finally become permanent faults. Until a fault has developed into the non-persistent state its existence is normally not suspected. Locating non-persistent faults presents great difficulty at present.
Many faults on low voltage cables are attributable to mechanical failure of the outer metallic sheath allowing moisture to penetrate into the insulation. The mechanical failure may occur as a result of deterioration due to ageing, but more commonly is the result of accidental contact during excavation work in the vicinity of the cable. The damage frequently is insufficient to cause the circuit protection, e.g. circuit breakers or fuses, to operate and therefore, unless the cable is subsequently exposed and the damage observed, many cables lie in the ground with the damage to their sheaths undetected. If the metal sheath of a cable is broken, it is usually only a matter of time before moisture begins to penetrate into the insulation and, in the case of paper insulated cables, a transitory fault appears.
During the transitory stage of the development of a cable fault, the moisture entering the cable increases the dielectric loss in the insulation adjacent to the point where the sheath is damaged, eventually resulting in breakdown when the critical point is reached where thermal runaway occurs. The breakdown normally results in an arc which lasts typically less than 5ms. During the arcing period, there is typically a voltage of 100 to 200 volts across the fault causing a transient disturbance in the waveform. The arc is extremely violent and removes, or dries out, the faulty insulation and vaporises some material from both the core and sheath of the cable. The insulation is now effectively sound and able to withstand normal supply voltage, and the arc after becoming deionised during the current zero does not restrike. Unless the fault level at the point of breakdown is fairly high, it is unlikely that the circuit fuse will operate and the transitory fault will occur unnoticed, -- except, possibly, for a slight dip in the lights.
The explosive and thermal energy liberated at the fault causes an increase in the extent of the sheath damage, thereby allowing moisture to penetrate more easily into the cable and precipitate further transitory breakdowns. The time interval between successive transitory breakdowns tends to decrease as the amount of sheath damage increases, and eventually enough insulation may be degraded by the moisture for the energy in one half-cycle to be insufficient to clear the debris completely, and breakdown will then occur on successive half cycles at a progressively reducing voltage.
Subject to fault levels and fuse characteristics, there will now be the possibility of a "fuse clearance" resulting in a non-persistent fault, -- that is a fault where supplies can be restored immediately by replacing the fuse. In some cases "fuse clearance" may not occur until the fault has developed further and the discontinuous arc has been replaced by an almost continuous a.c. arc, or even until ohmic conduction has commenced. The operation of a fuse in the later stages of the development of a fault does not necessarily mean that a permanent fault condition has developed, although once ohmic conduction occurs successful restoration of the supply to consumers by fuse replacement is unlikely, since in all probability the insulation will either be heavily carbonised or else a metal to metal weld will have formed. During the development of a fault it is possible that sufficient material may be removed for the faulty core to become open circuited, in which case the fault can be located relatively easily.
If a fault develops into a permanent short circuit fault there are a number of ways of locating it. In one technique, known as the Transgradient method (see British Pat. Specification No. 1140446), the peak voltages at various points along a cable are measured and recorded when fault current is flowing. The method is extremely useful for locating premanent faults, but if the fault exhibits any of the characteristics of a non-persistent fault, that is of non-sinusoidal current flow, the results will be unsatisfactory. To locate non-persistent faults, or to eliminate the possibility of error in locating unstable permanent faults, the measuring units are connected in the "sheath gradient mode". This connection involves the use of an auxiliary reference conductor which may be the street lighting core in a 5 core cable, but more often is a separate temporary overground wire.